A conventional loose tube type optical fiber cable includes an optical fiber, a plurality of loose tubes each housing the optical fiber and being made of a thermoplastic resin material such as PBT, a centered tension member disposed in the center of the plurality of the loose tubes, and a sheath disposed on the outer periphery of the plurality of the loose tubes. In the loose tube type optical fiber cable, the plurality of loose tubes are stranded and collected in the circumference of the centered tension member, for example, in the SZ direction. Further, the sheath covers the outer periphery of the plurality of loose tubes thus collected.
A well-known structure of the above sheath is one in which the sheath is extrusion molded so as to form a hollow shape which is circumscribes with the outer periphery of the plurality of loose tubes. Additionally, another known structure of the above sheath is extrusion molded so that a sheath material is embedded in between the plurality of collective loose tubes (hereinafter referred to as filled extrusion molding).
Recently, in Europe and the United States where the loose tube type optical fiber cables are mainstream, the followings are being worked on to improve the installation workability of optical fiber cables into ducts: (a) improving pneumatic feeding performance and (b) rendering optical fiber cables dry.
U.S. Pat. No. 6,912,347 discloses a structure of an optical fiber cable in which a cable core is formed by intertwining a plurality of loose tubes each housing optical fibers. Moreover, when a sheath is placed on the outer periphery of the cable core, the thickness of the above sheath is made constant. This renders the outer shape of the sheath the same as or similar to that of the plurality of loose cables in the cable core.
Regarding (a) improving pneumatic feeding performance, the optical fiber cable of U.S. Pat. No. 6,912,347 is characterized by the reduction in sheath friction and the wavy surface of the sheath. In other words, the sheath is made thin with its thickness being constant so that the stranded texture shape of the loose tube would be exposed to the surface. Additionally, this document describes a thin, light-weight structure capable of being wired to a microduct.
According to U.S. Pat. Nos. 6,205,277 and 6,681,071, a plurality of loose tubes each housing optical fibers are stranded together to form a core cable. Then, a waterproof tape is placed on the outside of the plurality of loose tubes in this cable core. Thereafter, a sheath is formed on the outer periphery of the waterproof tape by filled extrusion molding. That is, a part of the sheath is embedded in between the plurality of loose tubes. Moreover, in all the above-described optical fiber cables, a waterproof method between the cable core and the sheath involves placing a waterproof tape around the outer periphery of the loose tubes, to thereby work on the above-described point, (b) rendering optical fiber cables dry.
According to JP-A-9-138331, foamed polyethylene (foamed PE) is formed, as a sheath, on the outer periphery of stranded optical fibers by filled extrusion molding. In this case, foamed polyethylene has a low Young's modulus and it is directly extrusion molded on the outer periphery of optical fibers, so the adhesion between the optical fiber strand and the sheath is increased. Accordingly, a structure unlikely to be bent is achieved, so that (a) improving pneumatic feeding performance is achieved.
U.S. Pat. No. 4,976,519 discloses an optical fiber in which an inner sheath is formed on stranded optical fibers by filled extrusion molding, and further an outer sheath is placed on the outer periphery of the inner sheath. In other words, a two-layer sheath is formed on the optical fiber cable. The inner sheath is a low Young's modulus material and is configured so as to be embedded in between the optical fibers to serve as a buffer layer. On the other hand, the outer sheath achieves (a) improving pneumatic feeding performance.
According to U.S. Pat. No. 4,930,860, a thin film is placed on the outer periphery of stranded optical fibers so as to cover it, and on its outer periphery is formed a sheath by filled extrusion molding.    [Patent Citation 1]    U.S. Pat. No. 6,912,347    [Patent Citation 2]    U.S. Pat. No. 6,205,277    [Patent Citation 3]    U.S. Pat. No. 6,681,071    [Patent Citation 4]    JP-A-9-138331    [Patent Citation 5]    U.S. Pat. No. 4,976,519    [Patent Citation 6]    U.S. Pat. No. 4,930,860